OSAN AIR BASE, South Korea — Pentagon boss Jim Mattis arrived in South Korea on Friday to meet with the nation's top defense officials and American military commanders on the front line in countering North Korea's nuclear weapons program.

Mattis is emphasizing the Trump administration's push for a diplomatic solution to the problem. But he also has said the U.S. is prepared to take military action if the North does not halt its development of missiles that could strike the entirety of the United States, potentially with a nuclear warhead.

Making his second trip as defense secretary to the U.S. ally, Mattis will meet with South Korean officials as part of an annual consultation on defense issues on the Korean peninsula. He'll be joined in Seoul by the chairman of the U.S. Joint Chiefs of Staff, Gen. Joseph Dunford. President Donald Trump is scheduled to visit the city next month.

Two other developments Thursday showed the U.S. intention to continue building diplomatic and economic pressure on Pyongyang. The Trump administration imposed sanctions on 10 North Korean officials and organizations over human rights abuses and censorship, including a diplomat in China accused of forcing North Korean asylum seekers home.

Meanwhile, a rare military exercise involving three of the U.S. Navy's aircraft carrier strike groups was being planned for next month in the Asia Pacific, a U.S. official said. The likely exercise would happen around the time that Trump travels to the region.

Trump entered office declaring his commitment to solving the North Korea problem, asserting that he would succeed where his predecessors had failed. His administration has sought to increase pressure on Pyongyang through U.N. Security Council sanctions and other diplomatic efforts, but the North hasn't budged from its goal of building a full-fledged nuclear arsenal, including missiles capable of striking the U.S. mainland.

If Trump sticks to his pledge to stop the North from being able to threaten the U.S. with a nuclear attack, something will have to give - either a negotiated tempering of the North's ambitions or a U.S. acceptance of the North as a nuclear power.

The other alternative would be U.S. military action to attempt to neutralize or eliminate the North's nuclear assets - a move fraught with risk for South Korea, Japan and the U.S.

Michael Swaine, an Asia defense analyst at the Carnegie Endowment for International Peace, sees little chance the North will cave in to international pressure and give up its nuclear weapons.

"I just think the United States is far away from coming to grips with this problem in North Korea," Swaine said in an interview last week.

The U.S. has about 28,500 troops based in South Korea and has maintained a military presence there since the Korean war ended in 1953.

The sanctions aimed at North Korean officials and organizations was announced in conjunction with a new State Department report on dire human rights conditions in the isolated nation said to include extrajudicial killings, forced labor, torture, prolonged arbitrary detention and rape. The report, required by Congress, highlights abuses the U.S. says underwrite the North's nuclear weapons program, including revenues it derives from overseas laborers. Thousands of North Koreans are sent abroad every year to work in "slave-like" conditions, it says.

Among the seven officials and three entities put on the Treasury Department's Office of Foreign Assets Control blacklist Thursday were the commander of the Military Security Command, Jo Kyong-Chol; minister of labor, Jong Yong Su; and director of the external construction bureau, Kim Kang Jin. The bureau is the government agency that manages the construction firms that send laborers overseas. Also sanctioned was Chol Hyun Construction, a North Korean company that exports workers, primarily to Gulf states and Africa.

"We aim to send a signal to all DPRK government officials, particularly prison camp managers and mid-level officials, that we can and we will expose human rights abuses and censorship in the DPRK and that these individuals will suffer consequences for such actions," said Scott Busby, a senior official in the department's human rights bureau. DPRK stands for the Democratic People's Republic of Korea, the North's official name.

Those designated are blocked from holding any U.S. assets, and people in the U.S. are prohibited from having dealings with them. Busby could not confirm whether any of the designees have significant U.S. assets.

During a Pentagon briefing Thursday, Lt. Gen. Kenneth McKenzie, director of the Joint Staff, raised the prospect of the three carriers operating together at some point, but he provided no details. A U.S. official confirmed the plans for an exercise, but wasn't able to discuss the matter publicly so spoke on condition of anonymity.

Three Navy aircraft carriers and the ships that accompany them are currently thousands of miles apart in the Indian Ocean and the Pacific. But they are moving through the region and could be closer together in weeks.

The exercise plan has not been publicly announced, and officials would not say exactly where or when it would take place. McKenzie said the last time three carriers operated together was in 2007. At that time, it was for a naval exercise off Guam.

Mattis met with South Korean Defense Minister Song Young-moo earlier this week when the two attended a conference of Southeast Asian defense chiefs in the Philippines. Song told reporters there that going to war with the North must be viewed as a last resort.

In the Philippines, the 10-nation Association of Southeast Asian Nations declared their "grave concerns" over growing tensions on the Korean peninsula. They cited North Korea's testing and launching of intercontinental-range ballistic missiles as well as its September test of a nuclear device the North claimed was a hydrogen bomb.

Mattis pointed to the ASEAN ministers' statement as evidence of a broad consensus in Asia that North Korea is isolated and in violation of international norms.

U.S. government officials for decades have confidently but mistakenly predicted the approaching collapse of North Korea.

Twenty years ago, Mattis's predecessor five times removed, William Cohen, said as he peered into North Korea from inside the demilitarized zone at the two Koreas' border that Pyongyang's communist system was "decaying and dying." His view was widely shared in Washington.

Like others, Cohen underestimated the resilience of North Korea's ruling dynasty that started with Kim Il Sung. Kim Jong Un, the current ruler, assumed control of the country shortly after his father, Kim Jong-Il, died in 2011, and has accelerated the country's nuclear and missile programs.